Protective or surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives. The polymeric binder functions as a dispersant for the pigments, inerts, and other additives in wet coating compositions and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. Polymeric binders can be thermoplastic or thermosetting binders based on coreactive components such as a reactive functional polymer adapted to crosslink or coreact with a crosslinking component such as melamine, isocyanate, or epoxy.
Conventional thermosetting polymers often require high temperatures as well as external crosslinkers. Some crosslinkers, such as melamines in conventional industrial coatings or triglycidyl isocyanurate for powder coatings, can cause toxicity problems. Also, the release of volatile by-products, such as caprolactam, from some of these materials can cause film defects, such as cratering and bubbling.
It now has been found that excellent paint coatings can be produced based on a dual cure of esterifiable ionomeric polymeric binder comprising a carboxylic acid functional polymer of an epoxy-ester polymer which is at least partially neutralized with an organic zinc salt such as zinc acetate, zinc propionate, or a similar organic zinc salt, or the zinc inorganic salt of zinc carbonate to form an ionomer. In addition to ionomer formation, epoxy-esters can self-cure through an esterification reaction between excess carboxyl groups and hydroxyl groups on the epoxy backbone.
The epoxy-ester polymer was found to provide interreacting polymer chains which exhibit thermosetting properties comparable to coreactive polymeric binders crosslinked by an amino crosslinker. An ionomer can be defined as a polymer composed of a polymeric backbone containing a small amount of pendant carboxylic acid groups, usually less than 15 mole percent, which are neutralized partially or completely with an organic zinc salt or zinc carbonate to form an ionomer. These ionic moieties and their interactions dominate the behavior of the polymer itself where it is believed that the zinc ions are exchanged for a hydrogen ion of the polymer carboxyl group. Ionic hydrocarbon polymers for elastomers or plastics are disclosed in U.S. Pat. No. 3,264,272.
In accordance with this invention, changes in the properties of polymeric binders for paint coatings can be achieved through the introduction of zinc metal ions by way of an organic zinc salt or zinc carbonate where modification of polymer properties is believed due to aggregation of ions. For example, the coreaction of two ion pairs on adjacent polymer chains results in a four-centered aggregate which behaves essentially like a crosslink. Among the dramatic effects that have been observed with paint compositions are increases in the moduli, increases in glass transition temperatures, and increases in viscosities. The formation of ionomeric clusters in protective surface coatings causes the acid functional polymers to behave like a phase-separated block copolymer. The use of ionomers in powder coatings is particularly advantageous since it permits control of the process such that the temperature during extrusion is lower than that of the volatilization temperature of the counterion (in the acid form). Thus, the ionomeric crosslinks are not formed to a significant extent during processing of the coating or application to a substrate, or during flow out and leveling to form a smooth continuous film. In this invention, the majority of the ionomer formation takes place during the bake cycle which comprises temperatures higher than the temperature where the salt will melt or solubilize and participate in an equilibrium reaction of the acid functional polymer. Baking and curing conditions can be controlled so that the ionomer formation occurs during baking where temperatures are adjusted to that of the volatilization temperature of the acidified counterion. The ionomeric formations can be observed in powder coatings especially as well as solvent borne coatings such as high solids coatings.
The unique properties of ionomers offer an alternate curing mechanism for solvent and high solids coatings generally, and powder coatings especially, with potential for lower baking temperatures, less films defects, a unique balance of physical properties, less toxic curing chemistry, thermal reversibility and lower cost. Thus, the addition of low levels of ionic zinc groups to acid functional epoxy-ester polymeric binders has been found to have dramatic effects on the physical properties of the polymeric binder. Incorporation of zinc organic salts or zinc carbonate, for instance, causes an epoxy-ester polymer to exhibit improved film properties such as solvent resistance and hardness. Viscometry has also supported the formation of ionomeric domains in carboxylic acid functional systems, but with ionic crosslinking, these properties are known to be thermally reversible. Useful carboxyl epoxy-ester polymers characteristically exhibit low polarity, high hydrophobicity, and low hydrogen bonding characteristics. Zinc ionomers generally resist water or humidity and produce coatings which are not water sensitive. In addition to ionic crosslinking, the carboxyl functional epoxy-esters have the capacity to self-cure through an esterification reaction cure. The excess carboxyl groups in the epoxy-ester react under normal curing conditions with hydroxyl groups on the backbone of the epoxy moiety to form ester crosslinks. This esterification cure can produce good paint properties by itself, but superior properties can be obtained by including a zinc salt, such that a dual-cure is produced. The dual ionomeric-esterification cure produces films with superior hardness, flexibility, solvent resistance and weathering resistance. Good weathering resistance is not commonly obtained with paints containing large quantities of epoxy and, consequently, good weathering was surprising and unanticipated.
When zinc carbonate is used as the neutralizing salt, the esterification and ionomer cure produces water and carbon dioxide as by-products, so the ionomeric-ester co-cure is a low toxicity cure mechanism. In addition, various quantities of zinc salt can be added to achieve a balance of properties so this dual-cure system provides the paint formulator with a wide paint formulating latitude. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.